sql动态sql给变量复值

Before we delve into these SQL concepts, note that I like to do all my development in SQL Management Studio. So if you want to follow along go ahead and open that up.

在深入研究这些SQL概念之前，请注意，我喜欢在SQL Management Studio中进行所有开发。 因此，如果您想继续前进，请打开它。

变数 (Variables)

Variables are extremely useful in SQL scripts. They offer the flexibility needed to create powerful tools for yourself. They are especially useful in dynamic SQL environments. They can, and are often used as counters for loops to control how many times you want the code inside the loop to run. A statement can use it to test the value of the data, or a stored procedure can use it to return a value.

变量在SQL脚本中非常有用。 它们提供了为自己创建强大工具所需的灵活性。 它们在动态SQL环境中特别有用。 它们可以并且通常用作循环的计数器，以控制您希望循环中的代码运行多少次。 语句可以使用它来测试数据的值，或者存储过程可以使用它来返回值。

We can use the DECLARE statement to declare one or more variables. From there we can utilize the SET command to initialize or assign a value to the variable. Here is a quick example:

我们可以使用DECLARE语句来声明一个或多个变量。 从那里我们可以利用SET命令来初始化变量或为变量赋值。 这是一个简单的示例：

DECLARE @MAXRECORD INT

You can declare multiple variables in the same DECLARE statement just separate them with a comma. Example would be:

您可以在同一DECLARE语句中声明多个变量，只需用逗号将它们分开即可。 例如：

/*Declare Variables*/
DECLARE @DBNAME NVARCHAR(100),@RECOVERYMODE NVARCHAR(100),@MAXRECORD INT,@CURRENTRECORD INT,@SQL NVARCHAR(MAX)

That seems simple enough.

这似乎很简单。

We have declared a variable called @maxrecord as an INT data type. Now we can use the SET operator to assign it a value. Like this:

我们已将名为@maxrecord的变量声明为INT数据类型。 现在，我们可以使用SET运算符为其分配一个值。 像这样：

SET @MAXRECORD = 10

We just assigned the INT variable @maxrecord with a value of 10. This can change throughout a SQL script, thus providing great flexibility.

我们刚刚为INT变量@maxrecord分配了值10。这可以在整个SQL脚本中更改，从而提供了极大的灵活性。

Think of a variable as a box that acts as a place holder for whatever value you want to put in it.

可以将变量想像成一个盒子，充当要放置在其中的任何值的占位符。

The SET statement can only be used on variable at a time. This can become cumbersome if you need to assign values to multiple attributes. But you do need to use multiple set statements.

SET语句一次只能在变量上使用。 如果您需要为多个属性分配值，则可能会变得很麻烦。 但是您确实需要使用多个set语句。

SET @MAXRECORD = (SELECT MAX(ROWNUM) FROM [#DBRecovery])
SET @CURRENTRECORD = 1
SET @SQL = ''

However you can use a select statement to make this process a little easier. (Largely depending on what you’re trying to accomplish). Here is a quick example of assigning values to variables in a select statement.

但是，您可以使用select语句使此过程更容易一些。 （很大程度上取决于您要完成的工作）。 这是在select语句中为变量分配值的快速示例。

SELECT @DBNAME = '['+[DBName]+']'
FROM [#DBRecovery]
WHERE [ROWNUM] = @CURRENTRECORD

动态SQL (Dynamic SQL)

I often think of Dynamic SQL as “code that thinks”. Although it’s not quite cognitive, it can be used to handle a lot of tasks that would likely be repetitive or have a high administrative overhead. The concept of Dynamic SQL is one that allows you to build a SQL statement or a full batch of SQL code and present it as a character string. Then you can take this string and execute it. SQL Server gives us a couple of options to execute character strings as T-SQL Code. One is via the simple EXECUTE command (I like to use “EXEC”) or the sp_executesql stored procedure.

我经常将动态SQL视为“思考的代码”。 尽管不是很容易理解，但是它可以用于处理很多可能是重复的或高管理开销的任务。 动态SQL的概念使您可以构建一条SQL语句或一整批SQL代码并将其显示为字符串。 然后，您可以使用此字符串并执行它。 SQL Server为我们提供了两种将字符串作为T-SQL代码执行的选项。 一种是通过简单的EXECUTE命令（我喜欢使用“ EXEC”）或sp_executesql存储过程。

So, again why would we ever want to use Dynamic SQL? One reason would be the automation of administrative tasks. Let’s say you have a non-production server with every limited space and you want to make sure that all the databases on your server are in simple recovery mode (to minimize the growth of the transaction logs). Rather than manually query the server to identify the databases that meet this parameter and then manually set them to the proper recovery model you can write a piece of code to execute periodically. This code can identify the databases that aren’t compliant with your set rules and make then make the change for you. Thus reducing your administrative overhead. This will save you time and allow you to focus on meeting real deadlines, projects and such.

那么，为什么我们还要使用动态SQL？ 原因之一是管理任务的自动化。 假设您有一台具有有限空间的非生产服务器，并且您想确保服务器上的所有数据库都处于简单恢复模式（以最大程度地减少事务日志的增长）。 您可以编写一段代码来定期执行，而不是手动查询服务器以识别满足此参数的数据库，然后将它们手动设置为正确的恢复模型。 该代码可以识别不符合您设置的规则的数据库，然后为您进行更改。 从而减少您的管理开销。 这将节省您的时间，并使您可以专注于满足实际的截止日期，项目等。

了解EXEC命令 (Understanding the EXEC Command)

The EXEC command was around before the more robust sp_executesql stored procedure. Rather than placing Unicode strings into one of the parameters, you simply place it in parenthesis after exec.

在更强大的sp_executesql存储过程之前，存在EXEC命令。 无需将Unicode字符串放入参数之一，只需将其放在exec后面的括号中即可。

EXEC (sqlstatement)

EXEC（sqlstatement）

You can even place the piece of code into a variable and put the variable in parenthesis. (Note that EXEC allows the use of both regular character strings and Unicode character strings as input).

您甚至可以将一段代码放入一个变量中，并将该变量放在括号中。 （请注意，EXEC允许使用常规字符串和Unicode字符串作为输入）。

了解sp_executesql存储过程 (Understanding the sp_executesql Stored Procedure)

This is a great built-in stored procedure for SQL Server. It allows you to use input and output parameters allowing your dynamic SQL code to be secure and efficient.  The Parameters not only serve a purpose for flexibility, but they also inhibit SQL Injection attacks since they appear as operands and not part of the actual code.

这是SQL Server的出色内置存储过程。 它允许您使用输入和输出参数，从而使动态SQL代码安全有效。 参数不仅起到灵活性的作用，而且还抑制SQL注入攻击，因为它们显示为操作数，而不是实际代码的一部分。

Also note that as long as the SQL statement itself remains the same, yet only the parameters change, the query optimizer will utilize the same cached execution plan.

还要注意，只要SQL语句本身保持不变，但是只有参数更改，查询优化器才会使用相同的缓存执行计划。

Let’s use the example above regarding databases and simple recovery model. I’m not going to write out the entire script for you here as it is beyond the scope of the article. But the snippets I provide should provide enough information to show how to use this method.

让我们使用上面有关数据库和简单恢复模型的示例。 我不会在这里为您写出整个脚本，因为这超出了本文的范围。 但是，我提供的摘录应提供足够的信息以显示如何使用此方法。

My goal is to find out if any databases are not in the simple recovery model. If they are, I want them to be put into the compliant mode.

我的目标是找出是否有任何数据库不在简单恢复模型中。 如果是这样，我希望将它们置于兼容模式下。

/*Insert Databases names into Temp Table*/
BEGIN TRY
DROP TABLE #DBRecovery
END TRY
BEGIN CATCH SELECT 1 END CATCHSELECT ROWNUM = ROW_NUMBER() OVER (ORDER BY sys.[databases]),DBName = [name],RecoveryModel = [recovery_model_desc]
INTO #DBRecovery
FROM sys.[databases]
WHERE [recovery_model_desc] NOT IN ('Simple')/*Declare Variables*/
DECLARE @DBNAME NVARCHAR(100),@RECOVERYMODE NVARCHAR(100),@MAXRECORD INT,@CURRENTRECORD INT,@SQL NVARCHAR(MAX)/*Initialize Variables*/
SET @MAXRECORD = (SELECT MAX(ROWNUM) FROM [#DBRecovery])
SET @CURRENTRECORD = 1
SET @SQL = ''/*BEGIN LOOP*/
WHILE @CURRENTRECORD <= @MAXRECORDBEGINSELECT @DBNAME = '['+[DBName]+']'FROM [#DBRecovery]WHERE [ROWNUM] = @CURRENTRECORD/*Build Command*/SET @SQL = 'ALTER DATABASE ' + @DBNAME + ' SET RECOVERY SIMPLE'EXEC sys.[sp_executesql] @SQL/*Next Record*/
SET @CURRENTRECORD = @CURRENTRECORD + 1END
DROP TABLE [#DBRecovery]

I like to take the criteria and place the databases that need to be altered into a temp table.

我喜欢采用标准并将需要更改的数据库放入临时表中。

SELECT ROWNUM = ROW_NUMBER() OVER (ORDER BY sys.[databases]),DBName = [name],RecoveryModel = [recovery_model_desc]
INTO #DBRecovery
FROM sys.[databases]
WHERE [recovery_model_desc] NOT IN ('Simple')

As you can see, I am using variables as they play an important role in creating dynamic SQL.

如您所见，我正在使用变量，因为它们在创建动态SQL中起着重要的作用。

DECLARE @DBNAME NVARCHAR(100),@RECOVERYMODE NVARCHAR(100),@MAXRECORD INT,@CURRENTRECORD INT,@SQL NVARCHAR(MAX)

I use SET to assign values to a few of the variables. In this case, I want the @maxrecord variable to know how many records in total we must loop through. The @currentrecord acts as our counter. Once @currentrecord is equal the @maxrecord the loop will end.

我使用SET将值分配给一些变量。 在这种情况下，我希望@maxrecord变量知道总共必须循环通过多少条记录。 @currentrecord充当我们的计数器。 一旦@currentrecord等于@maxrecord，循环将结束。

/*Initialize Variables*/
SET @MAXRECORD = (SELECT MAX(ROWNUM) FROM [#DBRecovery])
SET @CURRENTRECORD = 1
SET @SQL = ''

I use the @SQL variable to store the actual code that will execute. In this case, it is a simple command to change the recovery model of the database. Remember we already identified which databases are not in simple mode when we created the temp table.

我使用@SQL变量存储将执行的实际代码。 在这种情况下，更改数据库的恢复模型是一个简单的命令。 记住，我们在创建临时表时已经确定了哪些数据库不在简单模式下。

/*Build Command*/SET @SQL = 'ALTER DATABASE ' + @DBNAME + ' SET RECOVERY SIMPLE'  

Then finally, I use the sp_executesql stored procedure to execute the command that is stored in the @SQL variable.

最后，我使用sp_executesql存储过程来执行存储在@SQL变量中的命令。

EXEC sys.[sp_executesql] @SQL

As the loop moves onto the next record in the table the @DBNAME will store the name of a different database, thus having the command execute on a different database. This is an example of SQL code acting dynamically.

当循环移至表中的下一个记录时，@ DBNAME将存储其他数据库的名称，从而使命令在其他数据库上执行。 这是动态执行SQL代码的示例。

Another way to visualize a loop:

可视化循环的另一种方法：

翻译自: https://www.sqlshack.com/using-variables-in-dynamic-sql/

sql动态sql给变量复值